1. Field of the Invention
This invention relates to bending glass sheets and in particular bending on a bending shape or hollow configuration ordinarily designated as a frame or bending skeleton.
2. Description of the Related Art
The bending of glass sheets on a skeleton is a technique ordinarily used to bend glass sheets and in particular to bend simultaneously the two glass sheets intended to form a bent laminated glazing such as a transport vehicle windshield. In this technique, the two glass sheets laid on one another with insertion of a suitable separating agent are supported along their margins in an approximately horizontal orientation by a frame having the desired profile, i.e., a profile corresponding to the final one of the two bent glass sheets. Thus supported, the two glass sheets go into a bending furnace, generally a furnace having zones of different temperatures.
The first of these zones is a preheating zone in which the glass sheets are heated to a temperature of the glass close to the softening point. The next zone is the bending zone where the glass sheets brought to a temperature of about 600.degree. C. will gradually be bent by gravity to assume finally the shape of the frame. The glass sheets are then cooled and removed from the bending frame at the output of the tunnel furnace.
This bending process on a skeleton is fully satisfactory when the curves are not very pronounced and/or when the bending is essentially cylindrical. When the curves are more pronounced, and in particular in the vicinity of the periphery of the glazings, what is called a counterbending can occur at the corners of the glass sheets, generally at two of these corners, or even at the four corners according to the geometry of the glazing, i.e., its curves and also its cutout shape. The counterbending corresponds to an undesired inversion of the curve.
The bent glass sheets obtained then no longer meet the requirement imposed on the glazing to be able to be mounted in a car body window opening.
To eliminate the counterbending, in the case of the bending of a single glass sheet intended to be tempered thermally and which for this purpose is greatly heated, it has already been proposed to hold the edges of said sheet during the bending operation. This process is not entirely satisfactory, in particular in the case of the bending of several glass sheets simultaneously.